The ability of electronic systems to carry information is founded on the concept of modulation. Modulation is the act of varying a signal according to information. In the its simplest form, modulation may be the act of pulsing a signal on and off in accordance with digital data. For instance, continuous-wave (CW) radio transmission is literally decoded by sensing the presence or absence of a radio signal. Through the years, modulation techniques have evolved. Amplitude modulation (AM) is a modulation technique that varies the level of a signal according to an information stream. Radio waves can be amplitude modulated by an analog signal allowing them to carry time varying information such as audio or video.
Digital data is now being used to convey all sorts of information. Audio and video can be digitized and then communicated as digital data. Because of the recent trend toward the use of digital data, new modulation techniques have been developed to more effectively convey digital data. Many new modulation techniques are embodied in an apparatus called a “modem”. The term modem is an abbreviated acronym for the two terms “modulation” and “demodulation”. A modem can modulate a signal according to a stream of data and convey that modulated signal to a remote modem. The remote modem may then demodulate the signal in order to recover the original data stream that was used to modulate the signal.
Some modems now carry significant amounts of data by modulating more than one carrier signal. A digital data stream may be splintered into a plurality of sub-streams. Each sub-stream of data may then be used to modulate a particular carrier. At the receiving end, each carrier can be individually demodulated in order to recover a particular sub-stream of data. Once all of the sub-streams are recovered, a modem may then reconstruct the original data stream and deliver that to a particular destination.
Use of multiple carriers can lead to very complex hardware for either modulation or demodulation. A bank of signal generators would need to be cascaded with a bank of mixers. The resulting plurality of modulated carriers could then be combined and propagated to a remote modem. An equally complex filtering and demodulation circuit would need to be incorporated at the receiving end. The individual carriers would first need to be detected. Once they are detected, they can be demodulated in order to recover a sub-stream of data.
The availability of digital signal processing hardware has led to the refinement of digital modulation techniques. Using these techniques, a stream of digital data may be splintered into sub-streams. Each sub-stream of data may then be converted into a complex sample resulting in a frequency-domain, numerical representation of a modulated signal. By combining the complex samples derived from the individual sub-streams streams of data and then converting the samples into an analog signal, a plurality of individually modulated carriers may be generated. This is typically achieved by applying a numerical inverse-Fourier transformation of the samples into a time-domain numerical representation. The time-domain representation of the modulated carriers may then be converted into an analog signal using a digital-to-analog converter (DAC).
One problem with using multiple digitally synthesized carriers lies in the modulation shaping of the represented signals. Traditionally, time-domain representations of a modulated carrier have been defined using square-shaped pulses. The numerical representations of these modulated carrier signals exhibit a sin x/x response. As a consequence of this, the primary carrier is normally enveloped by exponentially decaying aliases, or side-lobes.
In order to ensure that the side-lobes of one carrier do not interfere with another carrier, the carrier frequencies may be selected so as to cause the carrier center frequency to coincide with the nulls in the other carriers and their sin x/x side-lobes. This frequency selection technique is called orthogonal selection. The method of splintering a data stream into a plurality of modulated carriers is called frequency division multiplexing. When used with orthogonal frequency selection, this type of modulation technique is called orthogonal frequency division multiplexing (OFDM).
The orthogonal frequency selection used in OFDM modulation is not entirely effective in preventing cross-carrier interference. Some energy from adjacent carriers may corrupt data carried by any particular carrier. This is especially true where the sampling frequencies used to convert analog signals into digital representations are not properly matched with the sampling frequencies used to generate the analog signal from a digital representation. In order to avoid this type of data corruption, the data carrying bandwidth of any particular carrier may be reduced. This means that the amount of data that could otherwise be carried by a plurality of carriers may need to be reduced to promote reliable signal transmission.
One very attractive alternative to reducing data bandwidth would be to eliminate, or at least attenuate any side-lobe artifacts resulting from the square shaped modulating pulse and its resultant sin x/x frequency-domain response. To do so would require that each modulated carrier be band-pass filtered to attenuate the side-lobes prior to conveying the carriers to a communications medium. The problem with such an implementation is the complexity of the analog filters that must be introduced into the signal path of the plurality of carriers used in multi-tone modems. Additional advantage could also be realized if a receiving modem could selectively isolate each carrier by first filtering out any spurious energy from the carrier pass-band. This, too, requires complicated hardware that is not only costly, but also generally unstable over time and temperature.
The ability of communication systems to carry information is founded on the concept of modulation. Modulation is the act of varying a signal according to information. In its simplest form, modulation may be the act of pulsing a signal on and off in accordance with digital data. Through the years, modulation techniques have evolved. Amplitude modulation (AM) is a modulation technique that varies the level of a signal according to an information stream. Radio waves can be amplitude modulated by an analog signal allowing them to carry time varying information such as audio or video.
Digital data is now being used to convey all sorts of information. Audio and video can be digitized and then communicated as digital data. New modulation techniques have been developed to more effectively convey digital data. Many new modulation techniques are embodied in an apparatus called a “modem”. The term modem is an abbreviated acronym for the two terms “modulation” and “demodulation”. A modem can modulate a signal according to a stream of data and convey that modulated signal to a remote modem. The remote modem may then demodulate the signal in order to recover the original data stream that was used to modulate the signal.
The bandwidth required to carry a modulated signal is typically proportional to the rate at which information is transmitted via that signal. Most modulation techniques work best when the channel over which the signal is transmitted introduces the same amount of attenuation at all frequencies, and also when the delay introduced by the channel is the same for all frequencies. When this is not true, the modem receiver implementation may require an equalizing filter that reverses the effect of the channel, so that the composite of the equalizer and the channel have the same attenuation and delay at all frequencies. Often the channel characteristics are not known until the signal is applied to the channel, in which case an adaptive equalizer is required that automatically adjusts to the channel characteristics. Equalizers generally add substantial expense to a modem implementation and they typically do not perform ideally, so that modem performance is to some extent compromised.
An approach that has become commonplace in the past 20 years to simplify the requirements of the equalizer is to use multi-carrier modulation (MCM). The principle behind MCM is that, rather than using a single high-rate modem whose signal occupies a wide bandwidth, a communication system splinters a high-rate data stream into some number of lowerrate data streams, also known as “substreams”. These lower-rate data substreams are used to modulate some number of corresponding carrier signals. In effect, each of these low-data-rate modulated carrier signals is a modem that occupies a much narrower bandwidth than a single, high-rate modem. The aggregate data rate of all these low-rate modems is made equal to that of the single high-rate modem. The advantage of using MCM is that the amount of variation in the attenuation and delay of the channel is typically much smaller over any narrow segment of bandwidth than it is over a broad section of bandwidth, so that if the rate of each of the low rate modems is low enough, an equalizer may not be needed at all.
If implemented in a brute force fashion, use of multiple carriers can lead to very complex hardware for either modulation or demodulation. A bank of signal generators would need to be cascaded with a bank of mixers. The resulting plurality of modulated carriers could then be combined and propagated to a remote modem. An equally complex filtering and demodulation circuit would need to be incorporated at the receiving end. The individual carriers would first need to be detected. Once they are detected, they can be demodulated in order to recover a substream of data.
It is well known in the art that the implementation of multi-carrier modems can be greatly simplified through the use of the Inverse Fast Fourier Transform (IFFT) at the transmitter and the Fast Fourier Transform (FFT) at the receiver. A stream of digital data at rate R may be splintered into N substreams, potentially of differing rates but with an aggregate rate of R. The bits in each substream of data may then be converted into a stream of complex samples corresponding to the constellation point (which may include either phase or amplitude information or both) used to modulate the carrier for that stream. When enough data has been input to the transmitter to form a constellation point for every carrier to be used by the transmitter, an IFFT is used to convert the set of constellation points into a set of time domain samples. These samples are then passed one at a time to a digital to analog converter (DAC), producing an analog waveform for transmission.
In some applications, it may be necessary that a communication channel can be used by multiple pairs of modems operating at the same time but using different frequencies, a technique called Frequency Division Multiplexing (FDM). For FDM to make efficient use of the available spectrum, the modulation system must have two characteristics. The first of these characteristics is that the transmitter must confine its transmitted signal to the greatest extent possible to the minimum bandwidth that it needs for reliable communication. Any energy that the transmitter produces in the band that another modem pair is attempting to use will create interference to the communication between those modems.
The second characteristic is that the modem receiver must be able to reject signals that lie outside of the bandwidth occupied by the signal it is trying to receive. Any energy from a modem pair operating in a different frequency band that is not rejected by the receiver will interfere with the receiver's ability to reliably receive the data intended for it. Typically both the transmitter and the receiver issue are addressed by the application of filtering for spectral containment at the transmitter and filtering for selectivity at the receiver. For example, in single carrier systems such as satellite communications, it is common practice in the art to use a Root Raised Cosine digital filter at both the transmitter and the receiver. The RRC filter typically requires some amount of excess bandwidth—that is extra bandwidth beyond the minimum theoretically achievable bandwidth—but the filter can provide outstanding spectral containment and selectivity.
The performance requirements relative to these two characteristics become even more difficult when the system includes a “near/far” scenario. In the near far scenario, a receiving modem is attempting to recover data from a transmitter whose signal encounters substantial attenuation on its propagation path (which is sometimes, but not always, the result of the signal having to travel a long distance—hence the term “far”) at the same time that a second transmitter is operating on a different frequency but whose propagation path to the receiving modem exhibits relatively small attenuation (possibly because it is physically near to the receiving modem). In this case, the required attenuation of the transmitter out of band emissions is increased by an amount equal to the path loss imbalance that exists between the two propagation paths. Similarly the requirement for receiver selectivity is also increased by the same amount. In systems where there can be 40 to 50 dB of path loss imbalance, supporting modulations that require SNR on the order of 30 to 40 dB requires out of band emissions that are 70 to 90 dB down, with similar requirements for receiver selectivity.
When used in an FDM application, a problem with using multiple digitally synthesized carriers has been that cost effective methods for application of the sorts of filter techniques used in single carrier modems have not been known. In a single carrier modem, the filter is most commonly placed prior to the mixer that converts the baseband sample stream to its carrier frequency. The analogous position in an IFFT based MCM transmitter is prior to the IFFT, but this does not work because to use filter requires an increase in the sample rate that is incompatible with the operation of the IFFT. The complementary problem exists on the receiver side. Thus in the prior art it has seemed that to implement MCM with filtering would require that the design use a brute force approach to implementing all the carriers and forego the simplification provided by use of the IFFT/FFT.
In the absence of filtering, each carrier is modulated by a train of rectangular time domain pulses. The resulting spectrum on each carrier exhibits a sin(x)/x response. Similarly, the receiver filter also has a sin(x)/x response. This shape is unacceptable in any system that requires good spectral containment in the transmitter and good selectivity in the receiver because its spectral skirts fall off only as the reciprocal of the frequency separation from the band edge.
MCM systems without filtering have still proved to be very useful because the carrier frequencies may be selected so as to cause the carrier center frequency to coincide with the nulls in the other carriers and their sin x/x side-lobes. This allows the receiver to recover the data used to modulate each carrier because at each exact carrier frequency all the interference from all the other carriers goes to zero. This frequency selection technique produces a version of MCM that is called often called Orthogonal Frequency Division Multiplexing, or OFDM
The orthogonal frequency selection used in OFDM modulation is not entirely effective in preventing cross-carrier interference. Some energy from adjacent carriers may corrupt data carried by any particular carrier. This is especially true when the delay of the channel is not the same at all frequencies across the band. In order to avoid this type of data corruption, the data carrying bandwidth of any particular carrier may be reduced. This means that the amount of data that could otherwise be carried by a plurality of carriers may need to be reduced to promote reliable signal transmission. Moreover, it is not possible to have an independent second transmitter operating on a different set of carriers and also using OFDM with rectangular pulse shaping, because any frequency and timing difference between the two transmitters will shift the carriers so that the property wherein all potentially interfering carriers have nulls at the desired carrier frequencies.